The invention concerns a method for manufacturing a container for transportation and storage of a radioactive material as well as a container within which a radioactive material can be transported and stored.
In the past, such containers became of great importance in the form of so-called xe2x80x9cCastor containersxe2x80x9d. They serve for transporting radioactive material, for example spent fuel elements from nuclear reactors, from the power plant to an interim storage or ultimate waste disposal site.
Sometimes, long distances have to be covered in that. Such a transportation requires an extremely high degree of security. This is true not only for the carrier vehicles (trucks, trains, ships) but also, above all, for the containers in which the fuel elements are transported, for example.
Above all, that involves two security aspects:
1. The container has to be constructed to prevent reliably an escape of radioactive radiation and gases.
2. The container has to be designed in such a manner that the security according to 1. persists even if an accident occurs, for example the container falling down from a carrier vehicle.
In that, the demands on the radioactive screening of the container are as great as on the strength and stability thereof.
Based on these aspects, it is an object of the invention to provide a method for manufacturing an appropriate container and a container meeting the demands mentioned above.
Among the radioactive beams, there are alpha rays, beta rays, gamma rays and neutron beams. Generally, alpha and beta rays have short ranges so that small material thicknesses (in the order of several millimeters) are sufficient to screen them. Therefore, the main thing in projecting a radiation protection container is the attenuation and absorption of the neutron and gamma radiation.
It is known in this context that the mass and thus the bulk density of an appropriate container wall is an important property.
In so far, steel containers as the mentioned Castor container were used in the past. Besides, so-called containers of steel/armoured concrete are known, which are constructed of a combination of steel/concrete.
The invention is based on the knowledge that the screening effect of such containers of steel/armoured concrete can be obtained by a special selection of a heavy concrete between steel walls.
The invention in its most general embodiment proposes a method for manufacturing a container for transportation and storage of radioactive material, having the following features:
an inner tube of metal is placed into an outer tube of metal in such a manner that an annular gap of a constant width is formed between the inner and outer tubes,
the annular gap is then filled with an aggregate or a mixture of aggregates, the minimum grain size of which is 2 mm and the maximum grain size of which is 20 mm, at least 95% by wt. of the aggregate having a bulk density greater than 4.2 g/cm3,
afterwards, a suspension of cement, water and a liquefier is injected under high pressure into the annular gap through at least one opening at the bottom end of the inner and/or outer tube until the suspension reaches the upper end of the outer tube in filling the gores existing between the aggregate totally,
the suspension of cement, water and liquefier being adjusted in such a manner that the concrete being formed (together with the aggregate) has a bulk density greater than 4,100 g/cm3 and a compressive strength of concrete according to DIN 1048, part 2 of  greater than 45 N/mm2 after 28 days.
The essential aspect of this method is the special technique for introducing the heavy concrete between the metal walls.
With a ready-made concrete mixture which would be filled into the annular gap, the required bulk densities and compressive strengths as well as the necessary screening from radioactive radiation could not be obtained.
This can be successful only by the selection of special aggregates which are filled into the annular gap in a first step and by the following injection of the cement paste under pressure, the filling degree of the cement paste being optimized substantially in that the injection is effected from the bottom to the top. In this way, an excellent and almost optimum filling of the gores between the aggregate parts can be effected and thus a dense high-strength concrete can be formed in the annular space.
Here, the term cement is used for all types of hydraulic binders. However, Portland cements are preferably used, that is Portland cements of the type CEM I 42.5 or with higher values (e.g. CEM I 52.5).
Aggregates having the required bulk density are for example barite, ferrophosphorus, magnetite, iron (steel), lead, hematite and granulated chill-cast iron as well as other metals, particularly heavy metals, the aggregates being able to be used individually or in mixtures.
A mixture of barite, ferrophosphorus, magnetite, hematite or mixtures thereof in combination with steel balls lead to very good values of density and compressive strength of the green concrete and the set concrete, respectively.
Various mixtures of aggregates have been tested in preliminary tests. Accordingly, mixtures of aggregates of barite, ferrophosphorus, magnetite, hematite or mixtures thereof having the grain fractions of 4 to 8 mm as well as 8 to 16 mm in combination with steel balls having a diameter between 4 and 10 mm show particularly favourable characteristics. The steel balls may have a spherical shape or be replaced totally or partly by lead balls or granulated chill-cast iron.
For example the quantities of the individual aggregate components may be as follows:
the aggregate of the grain fraction {fraction (4/8)}: 15 to 25% by wt.
the aggregate of the grain fraction {fraction (8/16)}: 15 to 25% by wt.
the steel balls having a diameter between 4 and 10 mm: 45 to 55% by wt.
As far as metal tubes have been mentioned above, this term particularly includes steel tubes, and here particularly steel tubes having a circular cross section, even though other shapes of cross section can be used as well, for example polygons.
An embodiment of the method provides to use an inner tube which is closed at its upper end and is shorter than the outer tube. In this case, the outer tube and the inner tube are placed on a base (a plate), for example, and then not only the annular space between the inner and outer tubes but also the space between the closed upper end of the inner tube and the upper edge of the outer tube is filled with the aggregate. Then, besides the annular space, the space between the closed end of the inner tube and the upper edge of the outer tube is filled as well with the suspension of cement/water/liquefier. In this way, a kind of xe2x80x9cconcrete coverxe2x80x9d is produced, which forms the container bottom in later application (after turning about 180xc2x0). Additionally, a plate of metal/steel may be secured to the upper edge of the outer tube, for example by screwing or welding.
The manufacturing method is simplified, if the inner tube and the outer tube are closed at their lower end with a cover of metal/steel before the aggregate is filled in. Preferably, this is done by screwing it onto the corresponding tube ends. In this way, a coaxial alignment of the inner and outer tubes is facilitated, that is also in filling-in the aggregate and in injecting the cement suspension, respectively.
This end being the lower container end in manufacturing the container forms the upper container end in the ready container (after turning about 180xc2x0). In this way, spent fuel elements may be inserted into the free space of the inner tube after screwing-off the steel cover, and then the container may be closed again.
The stability of the container is considerably improved if a reinforcement is inserted into the annular gap and into the space formed between the upper closed end of the inner tube and the open end of the outer tube, respectively, before the aggregate is filled in. The heat dissipation in the hydration of the cement is improved as well thereby.
Such a reinforcement may consist of a reinforcing cage extending substantially over the entire volume of the annular gap and said space, respectively.
As has been said that the cement suspension is injected under high pressure, this means first of all a pressure over 1 bar. With an increasing filling height in the annular gap and a correspondingly higher hydrostatic pressure, it is necessary to increase the injection pressure of the cement suspension as well, which may lead to an injection pressure up to 15 bars depending on the container height (for example 3 m).
Here, the width of the annular gap is assumed to be 20 to 30 cm, for example. The said xe2x80x9cbottom plate of concretexe2x80x9d may also have a corresponding thickness.
Because the density of steel is higher than the density of the heavy concrete, the container covers at the ends may have wall thicknesses which are a little lower, for example 5 to 15 cm.
As mentioned above, the invention also includes a container for transportation and storage of radioactive material, characterized by the following features:
The container consists of an outer tube of metal and an inner tube of metal being disposed therein with the same distance all around, an annular gap having a constant width being formed thereby between the inner and outer tubes,
the annular gap between the inner and outer tubes is filled with a heavy concrete consisting of an aggregate or a mixture of aggregates having a bulk density greater than 4.2 g/cm3, and a cement filling the gores between the aggregate, the heavy concrete having a bulk density of  greater than 4,100 g/cm3 and a compressive strength after 28 days according to DIN 1048, part 2 of  greater than 45 N/mm2, and
the outer tube and the inner tube being closed at the ends with a metal bottom and a metal cover, at least the metal cover being disposed removably.
In an embodiment, the container may be formed in such a manner that the inner tube ends at a distance from the lower end of the outer tube, is closed at this end, and a plate of heavy concrete exists between the closed lower end of the inner tube and the lower end of the outer tube, which is continuous in material with the heavy concrete in the annular gap.
This embodiment describes the container in usage condition. For manufacturing the inner and outer tubes are arranged in a condition turned at 180xc2x0, as described above.
According to the claimed method the heavy concrete may be reinforced, the reinforcement consisting of a reinforcing cage, for example.